There is in fact a neon bio-friendly spray paint you can get. You could mark every tree, log, stump, whatever. The problem is this is not a permanent solution, and flagging tape ends becoming garbage. I'm sure if your just playing with friends you could think up some clever barrier.

How about a moat!

__________________ďREALITY IS LIKE A STONE. TO MANY ITíS HARD AND COLD, THEY CANíT HUG IT OR EAT IT, IT ONLY FRUSTERATES THEM AND DOESNíT DO THEM MUCH GOOD. TO OTHERS ITíS STRONG AND DEPENDABLE, YOU CAN BUILD WITH IT, BUILD UPON IT, OR WORK WITH IT, ALSO USE IT TO SMASH PEOPLE IN THE FACE.Ē